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Even as some astronomers start shoveling dirt on the
much-heralded "first potentially habitable" alien world – a
planet called Gliese 581g – its co-discoverer is vigorously
defending the find.

Since a team of astronomers — led by Steven Vogt and Paul Butler—
announced the discovery of
Gliese 581g last fall, several research groups have conducted
studies casting doubt on the planet's existence. For a while,
Vogt kept a relatively low profile, hoping the debate would play
out in the scientific literature.

But that has not happened — and the doubting studies continued to
receive considerable media attention. So now, Vogt has decided to
speak out strongly in defense of Gliese 581g.

"In the past month, I have been contacted by reporters who have
noticed that the media coverage of our Gliese 581g result has now
turned decidedly pessimistic and personal," said Vogt, an
astronomer at the University of California, Santa Cruz. "It is
time to respond."

Making the find

Vogt and his team announced the discovery of Gliese 581g on Sept.
29. The planet, about 20 light-years from Earth, is the first
rocky, roughly Earth-size alien planet found to orbit its star in
the so-called "habitable
zone" — a just-right range that can allow liquid water, and
perhaps life as we know it, to exist.

The researchers also said they found another planet called Gliese
581f, located farther away from the host star. The two finds
increased the number of known planets in the Gliese 581 system to
six (four others were previously known).

Vogt and his team found 581f and 581g using the so-called radial
velocity — or Doppler — method, which looks for tiny wobbles in a
star's movement caused by the gravitational tugs of orbiting
planets.

To detect such subtle movements, the researchers looked at data
from two different instruments: the HARPS spectrograph, on a
telescope in Chile, and the HIRES spectrograph, on Hawaii's Keck
Telescope.

Doubting the find

The tantalizing alien planet discovery soon received a lot of
attention, from both the media and other researchers. One group
of astronomers, led by Michel Mayor of the Geneva Observatory in
Switzerland, performed a follow-up investigation in an attempt to
confirm the existence of Gliese 581g.

In December, a research group led by Rene Andrae of the Max
Planck Institute for Astronomy in Heidelberg, Germany, submitted
a paper asserting that Vogt's team incorrectly assumed the Gliese
581 planets have circular orbits.

And in January, Philip Gregory of the University of British
Columbia submitted a paper in which he re-analyzed the available
HARPS and HIRES data using Bayesian techniques — a different
statistical method than Vogt's team employed.

Gregory found no reliable signal from Gliese 581g and concluded
that the Gliese 581 system likely consists of four planets with
elliptical orbits, not six with circular ones.

These more recent studies have led some astronomers to conclude
that Gliese 581g most likely is not a real planet.

"It's a goner,"
planet-hunter Geoff Marcy, of the University of California,
Berkeley, told SPACE.com last month at the winter meeting of the
American Astronomical Society in Seattle. (Marcy was not involved
in the Gliese 581g discovery, or in the follow-up studies.) "It
doesn't exist."

Vogt said that he welcomes scrutiny and follow-up studies by his
colleagues as a vital part of the scientific process. But he
doesn't think that the critics have made a conclusive case
against Gliese 581g.

At the moment, Vogt added, his team is "confident that the
present HARPS + HIRES data set (as published by Vogt et al. 2010)
contains unambiguous and statistically defensible evidence" for
both 581f and 581g.

Vogt noted that the Swiss team reached its conclusion after
looking only at observations made by the HARPS instrument,
ignoring data from HIRES.

"We were very clear in our paper that the combined information of
both the HARPS and HIRES data sets was needed to detect and
characterize the very weak signals of planets f and g," Vogt
said.

While Vogt's team published its discovery of Gliese 581g in the
peer-reviewed scientific literature — standard research-vetting
procedure — the Swiss team has not yet submitted a paper, he
added.

"Until the Swiss HARPS team comes forward with a peer-reviewed
paper in the scientific literature that contains either new data,
or a new analysis, any claims they make against our findings have
no legitimacy," Vogt said.

Debates about statistics and assumptions

Vogt also takes issue with the follow-up study led by Andrae (of
the Max Planck Institute), which dings the discovery team for
assuming that the Gliese 581 planets all have circular orbits.

That's unfair, because his team did indeed consider other, more
eccentric types of orbits, Vogt said.

"We made it very clear in our paper that, while we presented an
'all-circular' solution, we explored literally hundreds of
non-circular orbit models, with one, or several, or all
eccentricities allowed to vary in the solution," Vogt said. "In
the end, we concluded that no eccentric solution was any better
than a simple 'Occam's Razor' model of all-circular orbits. Thus,
for brevity, we chose to simply show the all-circular case."

"With all due respect to Dr. Gregory's acknowledged long
experience with Bayesian techniques, I find several serious
weaknesses, indeed probably even flaws, in his methodology, that
have, I believe, led him to the wrong conclusion," Vogt said.

One of those weaknesses, Vogt said, is Gregory's decision to add
an extra uncertainty component to the HIRES data, giving it less
weight than the HARPS observations. And Gregory's analysis
doesn't properly take into account the gravitational effects the
Gliese 581 planets exert upon each other, Vogt added.

Further, Vogt said there is no reason to elevate Gregory's
Bayesian methods above his own team's statistical techniques. He
listed several exoplanet-hunting
cases in which, he claimed, Gregory's methodology fell short.

"Gregory's Bayesian Box has a largely unproven and rather
checkered record in practice, and therefore has not earned any
right to its claim to be the ultimate approach or last word in
the exoplanet modeling arena," Vogt said.

The future ofGliese 581g

To date, astronomers have discovered hundreds of alien planets –
the last confirmed count was around 520, with 1,235 more
candidates announced just this month. But Gliese 581g continues
to inspire further research, not all of which casts doubt on the
planet's existence.

For example, another researcher, Guillem Anglada-Escude of the
Carnegie Institution of Washington, submitted a paper last
October supporting the planet's existence. Anglada and a
colleague, Harvard's Rebekah Dawson, are working on another paper
that suggests 581g and 581f might both be out there, according to
Vogt.

The question of Gliese 581g's existence won't be settled
definitively until researchers gather more high-precision radial
velocity data, Vogt said. But as it stands, he's confident in his
team's data, and in the planet's existence.

"Over the past 15 years, our team has discovered and published
literally hundreds of planets, with not a single retraction or
proven false claim, and we are doing our level best to keep it
that way," Vogt said.